The two different ypes of pins as previously mentioned are the steering bypass pin, and the gear pins.

The steering bypass pin is inserted to disconnect hydraulic power ffor the steering from the nose wheel tiller, and the rudder pedals.

Not all planes have this. Even if they have it it is seldom used. It is common practice to disconnect trhe nose wheel steering torque linkage anytime the tow bar is connected. Some companies even remove hydraulic power to the system as further backup.

The gear pins are only inserted to perform maintenance functions on the airplane, and are not usually installed on a regular basis. However again it is some companies policy that whenever the plane is under tow the nose gear must be pinned.

So the pin that the marshall holds up is either the nose gear pin, or the steering bypass pin. depending on comapny policy.

At Air Canada, we used to use a nose down-lock pin to secure the nose gear from retracting while towing/push-back, however, this is no longer used for ramp operations. They used to be used on most aircraft types, however, the only a/c we use it on now is for towing operations with BAe-146's. I beleive maintenance still uses this pin during maintenance operations.
We do use a steering by-pass pin for all operations on all a/c (except B-737's which don't use any pins and BAe-146's).
As mentioned, this pin disconnects the hydraulics so the nose wheel can only be turned by the tow tractor and not with the steering tiller or rudder pedals.

On the 737 we finally decided that you could either insert a bypass pin to isolate the steering or shutdown the "A" hydraulic system. When I was at Alaska they did the latter, but a bypass pin was used on the MD-80's.

A lot of aircraft you don't worry about it, On the Metro as long as you keep the tires between the red lines on the NLG you are fine. A Lear has a steering deactivation switch, when it shuts down there are no turn limits. A CASA 212 has a pair of spring loaded pins on the nose gear scissors link that disconects the link. Then you can turn that nosegear through 360.